relationship question.

Okay so my boyfriend and I have been together 2 1/2 years. We have a 1 1/2 year old daughter. As you can tell we got pregnant super fast. Only had known each other a couple months before I was pregnant, 18 years old at the time. Well when I was about 5/6 months pregnant I had found out that he had cheated on me with his ex girlfriend. Before me they had been together about a year and a half and straight after they broke up we met and started to date. My question is, I know it was two years ago, and yes it kills me. But I knew what I was doing when I forgave him. It does still bother me at times and I do bring it up a lot,  he tells me that he's trying to move forward from it so why can't I? I've sat there and argued with him over and over about how it's not my fault that I can't move in its his for his actions. It can drive a girl crazy to think that someone at that time loved them but really didn't. He tells me he's sorry and that we were 18 at the time (now almost 21) he didn't know what he wanted, we had only been together so long, but that if he didn't want me now, or love me now, and knew what he wanted then he wouldn't be here trying to fix it everyday and trying to provide for our family. But yet still here I am not being able to put it past me. Am I wrong? (Not asking if I'm wrong for being cheated on, but wrong for not trying to put it behind me?)